Motorcycle Design for Handling (Continued)
by:shift1313

Rear Ends Under Acceleration

Ive already talked about rear wheels and tires, but the rear has so many different things that effect handling its ridiculous. You wouldnt think so but everything right down to the size sprocket you choose will affect your handling. Hopefully my crude drawing will help. I do plan on making a better drawing but I wanted to get this down first.

The geometry in my pic is measurements taken off of my bike and it is actually drawn to scale. I tried to label everything that was important so hopefully its all there. And ofcourse all of those points will move but its way to hard to explain every aspect. Im going to explain the stuff in the drawing and how it will affect handling and give a generality on what to expect.

Ill first start with the numbers
Chain Pull Angle 30degrees, this is the angle you get from the contact patch of your rear tire up to your pole of moments. The pole of moments is found by drawing a line where your chain is under tension along the top of your sprockets is and projecting that out until it meets the projection of your swingarm arm from your axle through your pivot point. It sounds more complicated then it is.
Weight Transfer Angle 23degrees, this angle is found by drawing a line from your rear tire contact patch and meeting it with an imaginary line from your front tire contact patch up to your horizontal center of gravity. In this case my CG is roughly 23.5" sitting an inch above my front tire. You dont need to know your exact numbers, they are really hard to get, but knowing close approximations will give you an idea of how your bike will handle under acceleration.

For the scope here it wont matter but the wheelbase in the pic is 56", the swingarm angle is now 11degrees and the chain pull on top is roughly 3degrees.

And now to explain what all that stuff means. I dont think its really important to understand all the physics behind it, if anyone wants to know id be more then happy to try and explain the best I can, but I think its more important to know the affects caused by a change in the angles. I will give 3 cases.

Case 1:
Chain pull angle and weight transfer angle are equal. This would happen if you had a really high center of gravity on a bike or possibly a really large rear sprocket. When you accelerate in this situation it will tend to rise the center of gravity of the bike. Unloading the front end as well as sqwating the rear end slightly. Its obvious if youve ever ridden a bike with a high CG that it will wheelie very easily. Because of this alot of energy is lost raising the bike and lowering your acceleration. Also a note that this will make it difficult to take a tight turn while accelerating.

Case 2:
Chain pull angle is less then the weight transfer angle. This setup, much like the first, will make the front end rise increasing your rake angle but at the same time will make the rear end squat a good bit. Just like the first case this will make accelerating in a turn very difficult. Even worse then the first case, this case will make your turn run wide. If youve ever ridden a high cg bike you know this to be true and it all makes sense if you think about the change in the rake angle explained earlier.

Case 3:
Chain pull angle is larger then the weight transfer angle. This is what my setup is modeled after. This makes the rear end rebound under acceleration. This means while the front end will raise under acceleration so will the rear end which keeps the bike from rotating like the other two cases. While the whole cg raises it doesnt change the rake angle making the danger of wider turns not an issue. This is how every sport/race bike will be setup so make sure you dont change something around and end up with a dangerous bike.

If you think about dirt bikes, they have much higher cg's then street bikes but the rider is a bigger part of the weight distribution. He can shift forward or backward making a huge change in the cg of the bike. This is why you can lean back while standing up and wheelie very easy but you have to lean way forward to accelerate. And also coming out of a turn you tend to "slide".

 

Swing Arm

More people will decided to build a swing arm before they build an entire frame. Ill try to note a few things that will hopefully be helpful when designing swingarms and setting up rear suspensions and I will talk about frame and swingarm materials and general construction in another post.

Single sided swing arm. This is a very very complicated piece to design. If you do not have a mechanical backround or some sort or design software I would stay away from making one of these. Because of its design it needs to be extremely stiff becauase it is always under a twisting load. For all intensive purposes the only real advantage is a quick tire change for endurance bikes. Practically it introuduces more problems then it solves and is really more for looks then anything.

Conventional Symetrical swing arm. This is obviously the most used and most popular for many reasons. Its easy to make and easy to make stiff. It can be made from anything from steel tubing to extruded aluminum box section tubing. I imagine if anyone tries to build a swing arm this is what they will build. I made a crude drawing to represent the load on a swing arm with dual shocks.

I think everybody understands the concept of moment, even if you dont know it. Think about a really tough bolt on your car that you cant get off with your wrench. Whats the first thing you do, grab a big pipe to fit over your wrench. This is basically a torque multiplier. The force you exert is still going to be the same but because of the distance from your bolt you are going to increase the amount of torque you are putting on it. With that being said you should notice the drawing has two different shock locations. The first one is going to be the most common. Putting the shocks close to your rear axle(the origin of the force) will reduce the amount of force put on the shock. This lets you run a softer shock, as well as a lighter swingarm since it doesnt have to absorb as much of a load. So this is important when you design a swingarm or even modify one to run dual shocks. You have to consider the original swingarm was designed to run a mono shock so the area closest to the pivot will be much stronger then the section by the axle. When designing shock mounts for a swingarm like this you should try to spread the load point out to as much of the swingarm as possible. In the bottom left of my drawing you will see this illustrated. If you load the center of a box tube swingarm i can guarantee that it will buckle under load. Instead if your mounting pieces span the entire top of the swingarm and over the edge your load will be transfered to the corners which are infinately stronger. Because you are introducting a vertical load you want to introduce it this way.

To the right of the picture you see two rough drawings of common swingarms. You can see from both that they are designed to be much stronger at the location of the shock then anywhere else. Again lets think about dual shocks for a second. We know that location of the shock mounting point can increase or decrease our load. One other thing to remember is this is on both sides. Your shock is only 1/2 of the total shock absorbtion on the rear. When you go to a single shock setup you are in effect doubling your shock load. With that being said designing a swing arm for this type of suspension will require much more structure around this area. If you look at most new bike swing arms they are triangular. They get larger as they get closer to the shock point and then decrease again towards the pivot. If you actually did a load diagram on this type of setup you would see close to a straight horizontal line. There are benefits to this setup though. You are able to get the weight of the shock closer to the CG of the bike. You can use a smaller shock with less travel because your linkage ratios are going to increase the amount of travel you can have. There are numerous reasons to use monoshock over dual but both solutions can work the same in the end. One major benefit to the mono over dual is the top shock mount. On a mono shock this is going to be at a very good structural point on your frame. On a dual shock this will be on your subframe which is usually not as strong and may cause unwanted frame noise from the suspension. One other quick thing to mention on swingarms is bracing. You will notice most bikes have bracing now but older bikes didnt have them. If you look at old gp race bikes they always added bracing to the arms. This is because the use of stickier tires introduced more twisting in the swingarm during a turn. Top brace or bottom brace doesnt really matter but transfering the load from the rear axle to the strongest point of your swingarm is the key. You will notice the bracing starts at the rear and goes to the shock mounting point more times then not.

Continued on next page